It has been found that certain food products cook and taste best when heated at a specific temperature for a set period of time. As a result, restaurants and food service establishments, especially franchise food chains, have instituted strict criteria for preparation of fried food products. Consequently, restaurants and food service establishments will often require a deep fat fryer which can operate and maintain specific heating parameters.
Deep fat fryers are widely used in commercial food vending establishments, such as fast food restaurants, to heat food products, such as potatoes, fish, chicken, or the like. Accordingly, desirable characteristics in a deep fat fryer include rapid heating, without significant overshoot, to an operator selected cooking temperature, accurate maintenance of the cooking medium temperature to a temperature within a narrow range around the operator selected cooking temperature, minimal wearing of heating element components, and safety features which prevent injury to the operator or damage to the fryer.
Prior art fryers known heretofore typically include a vat for holding a cooking medium, temperature selection means for inputting a desired cooking temperature for a food product, a heating element (e.g., a gas burner or electric element) for heating the cooking medium, temperature sensing means for sensing the temperature of the cooking medium, and a fryer controller for providing overall control of fryer operations. One significant function performed by the fryer controller is control of the heating element.
The heating element is operated in a melt mode, a post-melt mode, an idle mode, a cook mode, and a boil mode. In the melt mode, a "cold" cooking medium is heated at a slow rate to gradually introduce heat to the cooking medium. The post-melt mode quickly heats the cooking medium to reach an operator selected setpoint temperature (i.e., cooking temperature) which is optimum for cooking the food product. The cooking medium is maintained at a temperature around the operator selected setpoint temperature in the idle mode. In this mode, the fryer operates in a steady state before, during or after a cooking operation. In the cook mode, food product is introduced into the cooking medium, and depending on the load size, may cause a drastic drop in the temperature of the cooking medium. It is during this mode that the food product is cooked. In a boil mode, the cooking medium is removed from the vat so that a cleaning operation can take place. In this respect, water and detergent are introduced into the vat and heated to a predetermined temperature (e.g., approximately 195.degree. F.).
With regard to the idle mode, prior art systems employ several different approaches. A first approach is known as ON/OFF control. The heating element is either on or off, with no middle state. The heating element is ON when the temperature of the cooking medium is below the operator selected setpoint temperature, and OFF when the cooking medium temperature is above the setpoint temperature. A second approach is known as proportional control. The proportioning action occurs within a "proportional band" around the setpoint temperature. Outside this band, the controller functions as an ON/OFF unit, with the heating element either fully ON (below the band) or fully OFF (above the band). However, within the band, the heating element is turned on and off for short intervals, wherein the ratio of ON time to OFF time is varied based upon the difference between the cooking medium temperature and the setpoint temperature. A third approach is known as PID (proportional, plus integral plus derivative control). PID combines proportional control with two additional adjustments, which help compensate to changes in the system. Integral action produces an output that forces the temperature back to the setpoint thus eliminating the "droop" associated with "Proportional Only" control, and derivative determines how fast (i.e., the rate) the cooking medium temperature is changing.
One feature common to many prior art idle mode control strategies is that they attempt to minimize the peak-to-valley excursions of the cooking medium temperature. The peak-to-valley excursion is the range of cooking medium temperatures obtained around the setpoint temperature. The maximum temperature establishes the "peak," while the minimum temperature establishes the "valley." The peak-to-valley excursion of the cooking medium temperature is usually minimized by periodically pulsing the heating element, wherein the pulses have a fixed duty cycle. In this respect, the pulses of heat are intended to add the heat necessary to balance the heat lost to the surrounding environment.
There is often a need to quickly and accurately enter an idle mode, wherein a steady state operation is established. In this regard, there are many situations where it may be necessary or desirable to quickly increase the temperature of the cooking medium to a temperature at or above the setpoint temperature, and thereafter to maintain the cooking medium at a temperature around the setpoint temperature. For instance, this situation may arise when the cooking medium has failed to reach a temperature near the setpoint temperature and it is desired to begin a cooking operation. This situation may also arise when a first cooking operation has been completed, the cooking medium is at a temperature below the setpoint temperature, and it is desired to begin a second cooking operation. Moreover, this situation may arise during a cooking operation where, due to stirring of the cooking medium, the cooking medium temperature has dropped below the setpoint temperature. Therefore, the cooking medium may be at various temperatures below the setpoint temperature at the time in which it is desired to move to the setpoint temperature.
In view of the foregoing, there is a need for a fryer controller that can quickly and accurately move the temperature of the cooking medium from various temperatures below the setpoint temperature to a temperature at or above the setpoint temperature, and thereafter maintain the cooking medium at a temperature around the setpoint temperature.
The present invention addresses the foregoing and other problems, and is directed to an electronic control system and more specifically to an electronic control system having a programmable microcontroller and associated peripherals, for use with heating apparatus, such as fryers, ovens, pressure cookers, pasta cookers, holding cabinets, furnaces, and water heaters.